Switched supply for operational amplifier

ABSTRACT

There is provided a circuit and method for providing a supply voltage to an operational amplifier. A switch has a plurality of inputs connected to a respective plurality of supply voltages. An output of the switch is connected to a supply voltage terminal of an operational amplifier. The input of the switch is selected in dependence of the voltage levels to which a signal is to be amplified

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/096,984,filed Mar. 12, 2002, now U.S. Pat. No. 6,621,350, which is a divisionalof application Ser. No. 09/692,656, filed Oct. 19, 2000, now U.S. Pat.No. 6,400,228, which claims priority to Great Britain application no.0021438.7, filed Aug. 31, 2000, now United Kingdom Patent No. 2,366,461.

FIELD OF THE INVENTION

The present invention relates to a technique for providing supplyvoltages to a supply terminal of an operational amplifier.

BACKGROUND TO THE INVENTION

In certain applications the voltage levels to which an operationalamplifier is required to drive signals is variable. Thus, for example,the operational amplifier may at times be required to drive outputsignals to a voltage level of 15 volts, whilst at other times it mayonly be necessary to drive output signals to a voltage level of 5 volts.However in order to cover the full range of possible output voltagelevels, the operational amplifier is required to be provided with thevoltage supply corresponding to the highest voltage level, namely inthis example 15 volts.

It is therefore an object of the present invention to provide animproved technique for providing a supply voltage to a supply terminalof an operational amplifier.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention there isprovided a circuitry for providing a supply voltage to an operationalamplifier, comprising:

-   -   a switch having a plurality of inputs connected to a respective        plurality of supply voltages, and an output connected to a        supply voltage terminal of the operational amplifier, wherein        the input of the switch is selected in dependence on the voltage        level to which a signal is to be amplified. Thus, advantageously        the operational amplifier is only provided with a supply voltage        level corresponding to the highest voltage level to which it        requires to drive a signal, thereby saving power consumption        when only low voltage levels are needed.

A digital to analogue converter may receive digitised values, andgenerate a corresponding analogue signal for amplification by theoperational amplifier. The input of the switch may be selected independence on the digitised values. The input of the switch may beselected in dependence on either the peak digitised value of thedigitised values or the average digitised value of the digitised values.

The input of the switch may be selected by a control signal generated independence on the digitised values. The digitised values may be storedin the digital to analogue converter.

Preferably the switch has a first and a second input connected to arespective first and second supply voltages, a first supply voltagebeing lower than the second supply voltage, wherein the input of theswitch is selected to be the second input if the voltage level to whichthe signal is to be amplified exceeds a predetermined level.

The circuitry for providing the supply voltage to the operationalamplifier may further comprise:

-   -   a further switch having a plurality of inputs connected to a        respective plurality of further supply voltages, and an output        connected to a further supply voltage terminal of the        operational amplifier, wherein the input of the further switch        is selected in dependence on the voltage level to which the        signal is to amplified.

In a further aspect the present invention provides a method of providinga supply voltage to an operational amplifier, comprising the steps of:

-   -   providing a plurality of supply voltages; selecting one of the        supply voltages in dependence on the voltage level to which a        signal is to be amplified; and connecting the selected one of        the plurality of supply voltages to a supply voltage terminal of        the operational amplifier.

The method may further comprise the step of converting digitised valuesinto an analogue signal for amplification by the operational amplifier.The step of selecting one of the supply voltages may be dependent uponthe digitised values. The step of selecting one of the supply voltagesmay be dependent upon either the peak digitised value of the digitisedvalues, or the average digitised value of the digitised values.

The step of selecting one of the supply voltages may include generatinga control signal in dependence of the digitised values.

Preferably there is provided a first and second supply voltage, thefirst supply voltage being lower that the second supply voltage, whereinthe second supply voltage is selected if the voltage level to which thesignal is to amplified exceeds a predetermined level.

The method of providing the supply voltage to the operational amplifiermay further comprise the steps of:

-   -   providing a plurality of further supply voltages; selecting one        of the further supply voltages in dependence on the voltage        level to which a signal is to be amplified; and connecting the        selected one of the plurality of further supply voltages to a        further supply voltage terminal of the operational amplifier.

The circuitry for providing the supply voltage to the operationalamplifier, or the method for providing a supply voltage to theoperational amplifier, may be associated with an xDSL modem. A DSL is adigital subscriber line. The term “x” in front of DSL implies that theinvention is relevant to any type of digital subscriber line technology.For example, the invention is relevant to ADSL (asymmetric digitalsubscriber line) technology, the VDSL (very-high-data-rate digitalsubscriber line) technology, HDSL (high bit rate digital subscriberline) technology etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with regard to a non-limitingexample by way of reference to the accompanying drawings in which:

FIG. 1 illustrates a first embodiment of a circuitry for providing asupply voltage to an operational amplifier in accordance with thepresent invention; and

FIG. 2 illustrates a second embodiment of circuitry for providing asupply voltage to an operation amplifier in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following example of the present invention is described withspecific reference to an application in an ADSL (asymmetric digitalsubscriber line) implementation. It will be apparent to the personskilled in the art that the present invention is generally applicable toany operational amplifier implementation.

Referring to FIG. 1, the operational amplifier block according to thepresent invention as shown in this embodiment is generally designated byreference numeral 10. The block comprises an operational amplifier 14and a switch 34. For the purposes of describing the present invention inthis example there is also provided a digital to analogue converter 12and a logic gate 16. The digital to analogue converter 12 receivesdigital values on line 18, and generates an analogue signal on lines 20and 22 to the positive and negative inputs of the operational amplifier14 as is well known in the art. The operational amplifier 14 has a firstsupply voltage terminal 36 connected to a node 33 of the switch 34. Asecond supply voltage terminal 24 of the operational amplifier 14 isconnected to ground. The operational amplifier 14 generates amplifiedanalogue signals on line 26. A switch 34 has two additional nodes 40 and42. Node 40 is connected to a supply voltage V_(CCL) on line 30. Node 42is connected to a supply voltage V_(CCH) which is connected to line 32.An output of a digital to analogue converter 12 on line 28 is providedto logic gate 16 which in turn provides a control signal on line 30 tothe switch 34.

As can be illustrated by the arrows within the switch unit 34, theswitch 34 is controllable to connect node 33 to either the node 40 or42. In this way the first supply terminal 36 of the operationalamplifier receives either the supply voltage V_(CCL) on line 30 or thesupply voltage of V_(CCH) on line 32.

The digitised values provided on line 18 to the digital to analogueconverter 12 are indicative of the voltage levels to which the analoguesignal on the output on line 26 of the operational amplifier are to bedriven to. Thus, these values are indicative of the supply voltagelevel, either V_(CCL) or V_(CCH), which the first supply terminal 36 ofthe operational amplifier should be connected to. Thus the logic gate 16will generate a signal on its output 30 in dependence on the value ofline 18 provided to the digital to analogue converter 12 meeting certainpredetermined conditions. Thus the logic gate 16 may be configured todetect a digitised value above a certain threshold value, and responsivethereto may set the signal on line 30 to switch to the higher supplyvoltage V_(CCH).

Alternatively, rather than the logic gate 16 being configured to switchto a high supply voltage responsive to a single peak being detectedabove a threshold level, the logic gate 16 may be adapted to accumulatethe digitised values and determine the average value thereof, and onlyresponse to the average value being above a peak is the control signal30 set to switch to the higher supply voltage V_(CCH).

Referring to FIG. 2, there is illustrated a second exemplary embodimentfor controlling an operational amplifier according to the presentinvention. Like reference numerals are used to denote elementscorresponding to elements shown in FIG. 1.

Thus, as can be seen from FIG. 2, the operational amplifier block,designated by reference numeral 11, is modified to include a furtherswitch 50 for controlling a supply voltage provided to the second supplyvoltage terminal 24 of the operational amplifier 14. The second supplyvoltage terminal 24 of the operational amplifier 14 is connected to anode 56 of the switch 50. The switch 50 additionally comprises two nodes52 and 54. The node 52 is connected to the supply voltage level V_(SSL)on line 58, and node 54 is connected to a supply voltage level V_(SSH)on line 60. In addition the switch 50 receives the control signal online 30 from the logic gate 16.

The operation of the general circuit of FIG. 2 is exactly the same asFIG. 1, with the exception that the control signal 30 now additionallycontrols the supply voltage applied to the second supply voltageterminal 24 of the operational amplifier. Thus, by way of example, thevoltage level of V_(CCL) may correspond to plus five volts, and thevoltage level of V_(SSL) correspond to minus five volts. In normaloperation, the switches 34 and 50 may have a default setting in whichthe supply voltage terminal 36 is connected to the voltage V_(CCL), andthe supply voltage terminal 24 is connected to the voltage V_(SSL.) Inthis example, it may be considered that the voltage level V_(CCH) isplus fifteen volts and the voltage level V_(SSH). is minus fifteenvolts. Responsive to the appropriate peak or average levels beingdetected in the digitised signals by the logic gate 16, the line 30 isset such that the switch 34 switches to connect the voltage V_(CCH) thesupply voltage terminal 36, and the switch 50 switches to connect thesupply voltage terminal 24 to the supply voltage level V_(SSH).

Thus it can be seen that with the use of the present invention power canbe conserved by only applying the high voltage levels to the operationalamplifier when it is required to amplify a signal to a higher voltagelevel.

1. Circuitry for providing a supply voltage to an operational amplifier,comprising: a switch having a plurality of inputs connected to arespective plurality of supply voltages, and an output connected to asupply voltage terminal of the operational amplifier; and a digital toanalogue converter for receiving digitised values and for generating acorresponding analogue signal for amplification by the operationalamplifier; wherein an input of the switch is selected in dependence onan average digitised value of the corresponding analogue signal and avoltage level to which the corresponding analogue signal is to beamplified.
 2. Circuitry according to claim 1, wherein the input of theswitch is selected by a control signal generated in dependence on thedigitised values.
 3. Circuitry according to claim 1, wherein thedigitised values are stored in the digital to analogue converter. 4.Circuitry according to claim 1, wherein the switch has a first input anda second input connected respectively to a first supply voltage and asecond supply voltage, the first supply voltage being lower than thesecond supply voltage, wherein the input of the switch is selected to bethe second input if the voltage level to which the signal is to beamplified exceeds a predetermined level.
 5. Circuitry according to claim1, further comprising: a second switch having a plurality of inputsconnected to a respective plurality of second supply voltages, and anoutput connected to a second supply voltage terminal of the operationalamplifier, wherein an input of the second switch is selected independence on the voltage level to which the signal is to be amplified.6. An xDSL modem including circuitry according to claim
 1. 7. A methodof providing a supply voltage to an operational amplifier, comprisingthe steps of: providing a plurality of supply voltages; convertingdigitised values into an analogue signal for amplification by theoperational amplifier; selecting one of the plurality of supply voltagesin dependence on an average digitised value of the analogue signal and avoltage level to which the analogue signal is to be amplified; andconnecting the selected one of the plurality of supply voltages to asupply voltage terminal of the operational amplifier.
 8. The method ofclaim 7, wherein the step of selecting includes generating a controlsignal in dependence on the digitised values.
 9. The method according toclaim 7 in which there is provided a first supply voltage and a secondsupply voltage, the first supply voltage being lower than the secondsupply voltage, wherein the second supply voltage is selected if thevoltage level to which the signal is to be amplified exceeds apredetermined level.
 10. A method of providing a supply voltage to anoperational amplifier according to claim 7, further comprising the stepsof: providing a plurality of second supply voltages; selecting one ofthe plurality of second supply voltages in dependence on the voltagelevel to which the signal is to be amplified; and connecting theselected one of the plurality of second supply voltages to a secondsupply voltage terminal of the operational amplifier.
 11. A method ofproviding a supply voltage to an operational amplifier of an xDSL modemaccording to claim
 7. 12. A method of providing a supply voltage to anoperational amplifier, comprising the steps of: providing a first supplyvoltage and a second supply voltage; determining an average value of aninput signal to the operational amplifier; and connecting one of thefirst supply voltage and the second supply voltage to a first powersupply terminal of the operational amplifier in response to the averagevalue.
 13. The method of claim 12, further comprising the step of:converting the input signal from a digital form to an analogue form. 14.The method of claim 13, wherein the analogue form of the input signal isa differential form.
 15. The method of claim 12, further comprising thestep of: producing a control signal from the input signal.
 16. Themethod of claim 15, wherein the control signal determines which one ofthe first supply voltage and the second supply voltage is to beconnected to the first power supply terminal.
 17. The method of claim12, further comprising the steps of: providing a third supply voltageand a fourth supply voltage; and connecting one of the third supplyvoltage and the fourth supply voltage to a second power supply terminalof the operational amplifier in response to the average value.
 18. Themethod of claim 17, further comprising the step of: converting the inputsignal from a digital form to an analogue form.
 19. The method of claim18, further comprising the step of: producing a control signal from theinput signal.
 20. The method of claim 19, wherein the control signaldetermines which one of the third supply voltage and the fourth supplyvoltage is to be connected to the second power supply terminal.